1. Field of the Invention
The invention relates generally to serial attached SCSI (SAS) device interconnections and more specifically relates to methods and apparatus for using either electrical or optical transceivers for such SAS device interconnections.
2. Discussion of Related Art
SAS standards include a detailed specification of the electrical interconnect utilized between two SAS devices. Electrical signaling standards are defined for a variety of transmission rates between devices including, at present, up to 6 GB per second data signaling. Further enhancements to SAS specifications envision 12 GB or higher speed in the relatively near future.
In the SAS signaling standards, out of band (OOB) signals are low-speed signal patterns that do not appear in normal data streams. OOB signals consist of a defined amount of idle time followed by a defined amount of burst time, the idle-burst pair repeated a specified number of times, and ending with a period of idle time. In accordance with the SAS (and serial advanced technology attachment-SATA) standards, in the idle period of the SAS OOB signal the electrical interconnection physical link carries a D.C. idle level—i.e., a differential 0V signal where the positive and negative differential signal paths are both driven toward ground potential. During the burst time of the SAS OOB signal the differential electrical physical link carries signal transitions represented as differential voltages driven on the positive and negative signal paths between the transceiver transmission transceiver and the receiving transceiver—thus presenting a level of energy to be received by the other side.
A variety of OOB signals are differentiated based on the duration of the idle time between the various burst times and other timing aspects of the idle and burst signaling periods. SAS and SATA specifications include a variety of such OOB signals including, for example, COMSAS, COMINIT, COMRESET, and, COMWAKE.
Despite the use of differential electrical signaling in accordance with the SAS specifications, as the transmission rates increase, cable length for the electrical signaling is limited. For example, at relatively high SAS transmission speeds electrical signaling cables may be limited to no more than a few meters of total length to avoid degradation of the signal quality.
Fiber-optic signaling capabilities are known to provide both high speed transmission and noise immunity over significant lengths of optical transmission media. However, in view of the requirements in the SAS standard for OOB signaling, it has been problematic to effectively utilize fiber-optic signaling for interconnection of SAS devices. In particular, current SFP (small form-factor pluggable) optical transceivers (including widely utilized quad small form-factor pluggable-QSFP) are incapable of utilizing present SAS standards for electrical OOB signaling. In particular, there is no optical midpoint or “D.C. idle” signal available in such SFP optical transceiver modules. In other words there is no equivalent signaling capability in SFP optical transceivers to represent an OOB idle time.
Some prior solutions have attempted to provide OOB signaling for SAS interconnect utilizing optical transceiver modules by encoding an idle time as the period of time that an optical laser is turned off (i.e., an optical laser off duration represents some analogous idle time duration in accordance with the SAS specifications). This prior technique presents other problems—the turn-on/turn-off time of high speed, low cost, SFP optical modules is several orders of magnitude slower than the timing requirements for OOB signaling in accordance with the SAS and SATA specifications. Further, problems arise in the receiving transceiver sensing loss of optical signal. The receiving transceiver sensing of a loss of optical signaling (RXloss) may impose further significant delays and distortion in the timing such that the desired OOB transmission may be corrupted.
Thus it is an ongoing challenge to provide simple, cost-effective, high-speed optical signaling between SAS/SATA devices that permit full compliance with the SAS/SATA standard including the OOB signaling capabilities.